The invention refers to a drive line which picks up power from a fluid flow power engine, such as a wind or water power machine, and transfers this to an electrical generator and which features a speed guide, a power impact reduction as well as a short-term energy storage facility.
Fluid flow engines which avail themselves of natural resources such as wind and water power, for the drive of an electrical generator make particular demands upon a drive line in terms of output transfer. It is typical that a heavily fluctuating output yield is available in the temporal flow process at the input shaft of the drive line. Additionally, there must be taken into account the particular problems involved in the characteristics and dynamics during the energy conversion of the kinetic energy of the fluid flow medium into the kinetic energy of the power intake, such as a wind power motor or a water power turbine. There thereby exists on the input shaft of a drive line for fluid flow power engines a system-inherent characteristic for the output conversion, which allocates a particular flow speed of the medium present (e.g. air and water) an optimal revolution speed/revolution momentum ratio according to the rapid-run count for the power intake, which, in its turn, is dependent upon the geometry and the design of the power input facility.
If it is the case that the drive line drives an electrical generator which feeds electrical energy into an electrical power supply grid, then it must be taken into account that the network frequency predominantly demonstrates a constant level. Fluctuations in the network frequency exist only to a very small extent since the dimension of this is drawn directly from the network itself.
The previously described requirements of a drive line are in particular in existence in the case of wind power plants. Here a variable power input is taken in by the wind power engine, in addition the wind power rotor has to feature a certain revolution speed which is dependent upon the wind speed in order to be able to extract optimal mechanical energy out of the air stream. In the following therefore there will be illustrated, using the example of a wind power plant, the problems involved in a speed-guided drive line with power impact reduction and short-term energy storage.
If, initially, the requirements made upon a drive line of a wind power plant are considered from the generator side, then an initial solution for the connection of an electrical generator onto the grid can be to design the entire drive line and thereby also the wind power rotor as fixed-speed. Such fixed-speed wind power plants can, when non-synchronous generators are being used, be connected in a simple form and manner to the voltage of an electrical power supply grid as electrical engines based upon the principles of conditional slip. Hereby the speed constancy will be conveyed to the drive line by the transmission on the wind rotor, so that the wind rotor does not travel at its optimum power output at varying wind speeds. It is a particular disadvantage of fixed-speed wind power plants that they, in particular when partially loaded, which is frequently the case with typical wind conditions, can only be operated with limited efficiency.
If a wind power plant in general, and in particular in the area of partial loading, is operated with variable speeds, then there arises the possibility of designing a drive line with either variable or constant output speed. Thereby, in both cases, the power output is also temporally changeable on account of the temporally varying momentum.
The first case leads to the use in wind power plant of frequency converters which motivate the generator with the required frequency or, respectively, provide compensations to the difference of the existing grid frequency and thereby make possible a variable-speed generator. This formulation however leads us away from the task herein illustrated and is particularly invested with difficulties, such as the complexity of the regulation and control circuits, the difficult to map parabolic characteristics of the wind rotor in the frequency converter, the stiffness of the defined generator characteristic curve by the frequency converter, of the low level of operational reliability in cases of high environmental burdens, a grid feed-in quality which can only be operated by extremely elaborate means such as e.g. low harmonic loading and the production of reactive volt-amperes.
The second case, namely to connect a variable rotor speed of the wind power plant with a constant generator speed without frequency converter, represents the topic here illustrated of a drive line for the transfer of a variable power with a variable input speed and constant output speed. The known solutions to this problem, in particular for wind power plant, deploy an overlay transmission which is used to split the mechanical power up into branches. In the case of variable-speed wind power plants there have become known two cases based upon this and which are used in order to keep the generator frequency at a constant level.
In the first system the input power is distributed via the overlay transmission between a large generator as well as a small servo-motor whereby it is generally the case that approximately 30% of the input power is relayed to the servo-motor. The generator is connected at fixed-speed to the grid or is fed via an auxiliary generator which is mechanically coupled to the generator. In order to stabilize the generator speed the servo-motor is either operated as a motor or as a generator with varying frequencies. In this kind of system the same problems exist as in the frequency-regulated generators.
In the second system, which works hydrostatically, instead of the electrical servo-motor hydraulic motors and pumps are used. Here also the problems arise of a difficult regulation characteristic, in particular of a laziness of response and relevant dead periods as well as pronounced non-linear features. Furthermore the hydraulic system components are disadvantageous on account of their elaborate design.
In addition to the previously described requirements for a drive line for fluid flow engines for connection to an electrical generator there arises in particular in connection with wind power plant the peculiarity that the tips of the rotor blades are not supposed to exceed a certain speed in order to reduce to a defined level the noise development which can be perceived to be disruptive. Depending upon the diameter of the wind power rotors, it is therefore necessary to limit their revolution speed to a certain maximum amount or, respectively, above a certain speed threshold, depending upon the wind-loading, to prescribe a speed sequence which, as near as possible, does not exceed a certain maximum level which however can vary, depending upon each location, for example whether an onshore or offshore location. In order to fulfill this stipulation a frequency converter on the generator can be used which imprints the required speed by means of its frequency on the generator and thereby limits the speed of the wind power rotor. This however requires the use of the solution explained above together with all of its disadvantages.
When using frequency converters the possibility arises that, where a substantially constantly sustained speed of the wind power motor via the variation of the torque relayed by the drive line, can also relay a variable power onto the generator, a power which is dependent upon the available kinetic energy of the airflow. It is however disadvantageous that, on account of the converter technology deployed, hitherto only a fixed speed guide along a prescribed nominal curve could be attained and thus it is in particular not possible to react to short-term fluctuations in wind flow. As a consequence, loading impacts resulting from gusts of wind cannot be compensated for by means of a short-term speed alteration and, as a consequence, have a direct effect on the generator and the mechanical structures. This is in particular to be regarded as being disadvantageous in respect of the loading accumulation and the operating period of the wind power plant connected thereto.